When a request is sent from a client terminal to a server in order to acquire a web page to be displayed on the client terminal, an HTTP session for transmitting the content of the web page from the server to the client terminal is established between the server and the client terminal. Typically, in the case of displaying a web page that contains many images on a client terminal, it is necessary to establish one HTTP session for transfer of one image. Because the number of HTTP sessions that can be established at a time in the server is finite, if further transfer of an image is needed when the number of established HTTP sessions has reached its limit, the transfer of the image cannot be carried out until an available session is created.
In view of the foregoing, a technique called CSS Sprites is known as a technique to reduce the number of HTTP sessions required for image transfer. In this technique, a plurality of images contained in one web page are combined together into one composite image, and the composite image is transferred to a client terminal. Because the composite image is treated as one image file, the number of HTTP sessions required for transfer of the composite image is one. The client terminal that has acquired the composite image cuts each image out of the composite image based on information contained in CSS and displays those images at specified positions in the web page. For example, the following Patent Literature 1 discloses a technique to reduce the size of the composite image created for CSS sprites.